In the field of driver devices, in particular in the field of LED drivers, high power factors and a safety isolated output are demanded. These drivers can either employ a PFC front end stage, with an isolating output stage or can use an isolating input stage that operates at a high power factor and which may employ a second non-isolated power stage. A corresponding driver device for driving LEDs is e.g. known from WO 2012/085759 A2.
Isolating PFC stages are usually formed as fly-back converters, which can provide high gain ratios which are needed for the respective PFC operation, however, the power density and the efficiency of fly-back converters is too low for applications that require miniature and/or efficient drivers, mainly due to the semiconductor losses in the switches and the rectifiers. Also the maximum power that is achievable with flyback converter designs is limited chiefly due to practical limits from the transformer.
Resonant converters which are also used for isolated PFC stages like the LC parallel resonant converters and the LCL converters—if operated close to or at resonance—have an input current proportional to the output voltage, which can in principle be utilized for PFC operation, however, the additionally needed isolation requires up to three electromagnetic components, which result in a low power density and high costs. Also class E converters provide high power factor operations, however, the power transistor has to withstand voltage stress that goes largely beyond limits of typical low cost devices.
As an isolating output stage, resonant LLC converters are widely used e.g. for professional LED drivers or for power supplies in consumer or office electronics equipment because of the high efficiency and the small size, however, the LLC converters have a limited gain ratio.